Programmable logic integrated circuit devices with low voltage differential signaling capabilities

ABSTRACT

A programmable logic device is equipped for low voltage differential signaling (“LVDS”) by providing an LVDS input buffer and/or an LVDS output buffer on the device. I/O pins on the device that are used together in pairs for LVDS can alternatively be used individually for other types of signaling. The LVDS buffers are constructed to give good performance and to meet LVDS specifications despite variations due to temperature, manufacturing process inconsistency, and power supply changes.

This application is a divisional of U.S. Pat. No. 6,535,031, filed asapplication Ser. No. 10/146,438 on May 13, 2002, which is a divisionalof U.S. Pat. No. 6,400,598, filed as application Ser. No. 09/843,299 onApr. 25, 2001, which is a continuation of U.S. Pat. No. 6,236,231, filedas application Ser. No. 09/340,222 on Jun. 25, 1999, which claims thebenefit of the following provisional applications: No. 60/091,524, filedJul. 2, 1998; No. 60/115,213, filed Jan. 8, 1999; and No. 60/115,214,filed Jan. 8, 1999. All of these prior applications are herebyincorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

A standard that has recently been developed for signaling over shortdistances is known as low voltage differential signaling (“LVDS”). Adescription of LVDS can be found, for example, in “LVDS Owner's Manual;Design Guide”, National Semiconductor, Spring 1997. (The referencementioned in the preceding sentence is hereby incorporated by referenceherein in its entirety.) Although LVDS is limited to distances of a fewmeters, this constraint is not a problem for use of this type ofsignaling between devices (e.g., integrated circuits) on a printedcircuit board or in other relatively compact systems.

Because programmable logic devices (“PLDs”) such as are shown in Cliffet al. U.S. Pat. No. 5,689,195 and Jefferson et al. U.S. Pat. No.6,215,326 are often desired as components of systems of the type forwhich LVDS is suitable, it would be desirable to provide PLDs with LVDScapabilities. (The references mentioned in the preceding sentence arehereby incorporated by reference herein in their entireties.) Inaddition, improvements are constantly being sought for LVDS circuitrygenerally, in terms, for example, of more uniform speed performancethroughout the permitted operating voltage range, improved rejection ofspurious signals, protection against open or short-circuited inputs,etc.

In view of the foregoing it is an object of this invention to provideimproved circuitry for LVDS generally.

It is another object of this invention to provide PLDs with LVDScapabilities.

SUMMARY OF THE INVENTION

These and other objects of the invention are accomplished in accordancewith the principles of one aspect of the invention by providing PLDswith input/output (“I/O”) pins that are connected in parallel to severaldifferent kinds of input and/or output buffers, including LVDS inputand/or output buffers. The PLD is programmable to allow any of the inputand/or output buffers to which an I/O pin is connected to be used. Thisallows the PLD to provide LVDS capabilities, if that is what is desired,without having to dedicate I/O pins to that particular type of use.Because an LVDS connection requires a pair of I/O pins, while many othersignaling protocols require only one I/O pin per connection, the PLDcircuitry is programmable to allow I/O pins to be used in pairs for LVDSor individually for other types of signaling.

To help make the speed of LVDS circuitry more uniform across theoperating voltage range permitted by the LVDS standard, circuitry isprovided for strengthening at least one of complementary current sourcesor sinks used in LVDS input buffers when the operating voltage is suchthat the circuitry associated with the other current source or sink isno longer able to help the input buffer operate. The thus-strengthenedcurrent source or sink helps to maintain the speed of the input buffereven though the circuitry associated with the other current source orsink is no longer operating effectively. Hysteresis circuitry may beprovided in LVDS input buffers to help the buffer reject spurious inputsignal fluctuations. Pull-up connections may be provided on LVDS inputsignal leads to help protect an LVDS input buffer from producingerroneous output signals in response to open or short-circuit conditionson those input signal leads.

An LVDS output buffer in accordance with the invention is constructed tohelp keep the output voltages within the LVDS standard or specificationdespite variations due to such factors as (1) manufacturing processinconsistencies, (2) temperature changes, and (3) power supply voltagefluctuations. The LVDS output buffer includes differential outputswitching circuitry connected in series via resistors between power andground potentials. One of the resistor circuits preferably includes acurrent source which tends to increase in resistance as the power supplypotential increases, thereby helping to counteract the effect ofincreasing power supply voltage. The transistors in the differentialoutput switching circuitry and the resistors in series with thatcircuitry are made so that they all have similar changes in resistancedue to manufacturing process variations and temperature changes. Thishelps keep the LVDS output voltages within LVDS specifications despitethese types of variations or changes. Capacitors are also preferablyincluded in the LVDS output buffer to improve the performance of thecircuitry in relation to switching transients.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic block diagram of a representativeportion of an illustrative embodiment of the invention.

FIG. 2 is a more detailed schematic diagram of an illustrativeembodiment of circuitry that can be used for a portion of what is shownin FIG. 1 in accordance with the invention.

FIG. 3 is a chart showing a representative operating condition of theFIG. 2 circuitry.

FIG. 4 is a more detailed schematic diagram of an illustrativeembodiment of circuitry that can be used for a portion of what is shownin FIG. 2 in accordance with the invention.

FIG. 5 is similar to FIG. 4 for another portion of what is shown in FIG.2.

FIG. 6 is a simplified schematic diagram of another representativeportion of an illustrative embodiment of the invention.

FIG. 7 is a simplified schematic diagram of circuitry of the type shownin FIG. 6 in conjunction with other circuitry, all in accordance withthe invention.

FIG. 8 is a simplified schematic block diagram of possible modificationof portions of earlier FIGS. in accordance with the invention.

FIG. 9 is a simplified schematic diagram showing possible combination offeatures from earlier FIGS. in accordance with the invention.

FIG. 10 is a simplified block diagram of an illustrative systememploying a programmable logic device including LVDS capabilities inaccordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a small portion of a programmable logic device 10constructed in accordance with this invention to include certain LVDScapabilities. FIG. 1 shows the circuitry required to enable device 10 toreceive LVDS signals. Circuitry for enabling device 10 to output LVDSsignals will be shown in later FIGS. and described later in thisspecification.

As shown in FIG. 1, device 10 has I/O pins 20. Although only two I/Opins 20 are shown in FIG. 1, it will be understood that these are onlyrepresentative and that device 10 may have many more such pins. Each ofdepicted I/O pins 20 can be used separately as an input or output pinwhen LVDS input via the depicted pins is not desired. In that case,external resistor 30 would not be present. Each pin 20 could thenreceive an input signal from the associated external signal lead 40, andthat signal would be applied to the logic of device 10 via theassociated conventional tri-state input buffer 50. For example, thenon-LVDS, single-conductor signaling being used could betransistor—transistor logic (“TTL”) signaling, and input buffers 50would then be TTL buffers. Any other single-conductor (non-LVDS)signaling can be used with appropriate input buffers 50. More than onetype of input buffer 50 may be connected to each I/O pin 20 so that anyof several single-conductor signaling protocols can be used. Each inputbuffer 50 is programmably controlled by an associated programmablefunction control element (“FCE”) 52 to be either on (i.e., able to drivean applied signal into the logic of device 10) or tri-stated (i.e.,off).

Each I/O pin 20 can be alternatively used as an output pin. In thatevent the associated tri-state output buffer 60 is enabled by itsassociated FCE 62 to apply a signal from the logic of device 10 to theassociated I/O pin 20, which applies that signal to the associatedexternal conductor 40. Again each output buffer 60 may be, for example,a TTL buffer or any other type of single-conductor signaling buffer, andseveral different types of such buffers may be associated with each pin20 so that any of several different output signaling protocols can beused.

If it is desired to use depicted pins 20 together for input of LVDSsignals, then all of FCEs 52 and 62 are programmed to disable theassociated buffers 50 and 60, and FCE 72 is programmed to enable LVDSinput buffer 70. Resistor 30 is also included across external leads 40in accordance with LVDS standards. LVDS input buffer 70 is therebyenabled to convert LVDS input signals on leads 40 to single-conductorsignals and to apply those signals to the logic of device 10.

From the foregoing it will be seen that depicted I/O pins 20 can eitherbe used individually for separate inputs or outputs (i.e., by using FCEs52 or 62 to enable input or output buffers 50 or 60, while using FCE 72to disable buffer 70), or the depicted pins 20 can be used together as apair for LVDS input (i.e., by using FCEs 52 and 62 to disable all ofbuffers 50 and 60, while using FCE 72 to enable buffer 70).

Incidentally, it will be recognized that another term frequently used inthe art for elements like buffers 50, 60, and 70 (and 600 in laterFIGS.) is “drivers”, and the term “driver” may therefore sometimes beused herein as an alternative to “buffer.”

An illustrative embodiment of LVDS input buffer 70 is shown in moredetail in FIG. 2. Buffer 70 is turned on by programing FCE 72 to turn oncurrent sink I1 and current source 12. When buffer 70 is not to be used,FCE 72 is programmed to turn off elements 11 and 12, thereby conservingpower that would otherwise be consumed. Buffer 70 is constructed to beable to detect LVDS signals over the full range of permissible voltagesaccording to LVDS standards. In particular, the offset voltage(“Voffset”) of the two LVDS input signals INA and INB in FIG. 2 can beanywhere in the range from 0 volts to 2.4 volts when Vcc (power or logic1 potential) for device 10 is 2.5 volts. Voffset is the average of thevoltages of INA and INB. In order to operate satisfactorily over such awide Voffset range, and especially at Voffset values that can be soclose to Vss (ground or logic 0 potential) or Vcc, buffer 70 includesboth an NMOS differential stage 100 and a PMOS differential stage 200.The NMOS differential stage includes NMOS transistors 110 and 112 andoperates except when Voffset is close to or below Vtn for NMOStransistors (i.e., the gate voltage required to turn on an NMOStransistor). Thus the NMOS stage stops operating or is partially turnedoff when Voffset is very close to ground potential. The PMOSdifferential stage includes PMOS transistors 210 and 212 and operatesexcept when Voffset is above Vcc-Vtp (i.e., the voltage below which thegate of a PMOS transistor must be in order for that transistor to turnon). Thus the PMOS stage stops operating or is partially turned off whenVoffset is close to Vcc.

Considering NMOS stage 100 in more detail, one LVDS input (INA) isapplied to the gate of transistor 110, and the other LVDS input (INB) isapplied to the gate of transistor 112. Current sink I1 pulls currentfrom the sources of both transistors 110 and 112. (Although in theparticular circuit relationship that it has in FIG. 2, element I1 isperhaps most accurately described as a current sink, the more genericterm for such an element is current source, and that term may sometimesbe used herein as an alternative for current sink.) PMOS transistor 120supplies current to the drain of transistor 110. PMOS transistor 122supplies current to the drain of transistor 112. PMOS transistor 130 isa current mirror for transistor 120. PMOS transistor 132 is a currentmirror for transistor 122. PMOS transistors 140 and 142 are relativelysmall transistors that increase the resistance of NMOS stage 100 toerroneous toggling in response to possible noise on LVDS signals INA andINB. In other words, transistors 140 and 142 help provide hysteresis inthe response of NMOS stage 100 to the LVDS input.

PMOS stage 200 has elements that are functionally analogous to severalof those described above for the NMOS stage. Current source 12 suppliescurrent to the sources of transistors 210 and 212. INB is applied to thegate of transistor 210 and INA is applied to the gate of transistor 212.NMOS transistor 220 conveys current from the drain of transistor 210.NMOS transistor 222 conveys current from the drain of transistor 212.NMOS transistors 240 and 242 provide hysteresis.

The combined differential outputs of NMOS stage 100 and PMOS stage 200are applied to the gates of NMOS transistors 310 and 312 in the outputstage 300 of buffer 70. Output stage 300 converts the differentialoutput signals to a single TTL output signal suitable for application tothe logic of device 10. Output stage PMOS transistor 320 suppliescurrent to the drain of transistor 310. Output stage PMOS transistor 322supplies current to the drain of transistor 312. The TTL output signalof buffer 70 comes from the drain of transistor 310.

To illustrate the operation of the FIG. 2 circuitry, FIG. 3 shows thecondition of various elements in FIG. 2 when Voffset is in a middlerange (i.e., not as low as Vtn or as high as Vcc-Vtp) and when INA is100 millivolts higher than INB for LVDS transmission of a signal havinga first polarity or logic value. (When INB is 100 millivolts higher thanINA for LVDS transmission of a signal having a second polarity or logicvalue, all of the conditions shown in FIG. 3 are reversed. For example,transistors shown as “partially on” in FIG. 3 become “on”, and theassociated transistors shown as “on” in FIG. 3 become “partially on”.“On” transistors 320 and 322 in FIG. 3 also become “partially on.”) Asis at least implied by the earlier discussion, when Voffset becomesapproximately Vcc-Vtp or higher, PMOS differential stage 200 ceases tooperate, but NMOS stage 100 continues to perform well and provides theproper logic output on the right in FIG. 2. Thus the LVDS input buffercircuitry shown in FIG. 2 operates well over the entire Voffset requiredby the LVDS standard.

FIG. 2 also shows optional NMOS transistors 410 and 412 for respectivelyproviding a weak pull down of leads INA and INB to ground. This isdesirable for helping to prevent buffer 70 from producing spuriousoutput signals in the event that leads INA and INB are open or shortedexternal to device 10.

An especially preferred embodiment of current sink I1 in FIG. 2 is shownin FIG. 4, and a similarly preferred embodiment of current source 12 inFIG. 2 is shown in FIG. 5. The current sink of FIG. 4 has the advantagethat as Voffset approaches and possibly exceeds Vcc-Vtp, the currentdrawn by the current sink increases. This increases the strength of NMOSdifferential stage 100 in FIG. 2 as the strength of PMOS stage 200 isdecreasing (or even becoming a load). In this way the speed performanceof LVDS buffer 70 is kept relatively constant even when PMOS stage 200ceases to contribute.

Considering FIG. 4 now in more detail, the current sink circuitry ofthis FIG. is turned on by programming FCE 72 to turn off NMOStransistors 502 and 504. Conversely, programming FCE 72 to turn ontransistors 502 and 504 turns off the depicted current sink by clampingthe gates of all of NMOS transistors 530, 532, 540, and 542 to ground,thereby turning off all of those transistors. The following furtherdiscussion of the FIG. 4 circuitry assumes that the current sink isturned on by turning off transistors 502 and 504.

I1 is the current drawn by element I1 in FIG. 2. Vref is a referencepotential having a value which is approximately one-half of Vcc. ThusPMOS transistors 510 and 512 are on at all times. If INA and INB arehigher than Vcc-Vtp, both of PMOS transistors 520 and 522 are off. Thismeans that both of NMOS transistors 530 and 532 are off. (Transistors530 and 532 are connected in a current mirroring configuration.) Becausetransistor 530 is off, all current from transistor 510 must flow throughNMOS transistor 540, which is on. NMOS transistor 542 is connected in acurrent mirroring configuration with transistor 540. Because transistor540 is strongly on, transistor 542 will be strongly on and current I1will be relatively large.

Values of INA and INB below Vcc-Vtp cause transistors 520 and 522 toturn on. This turns on transistors 530 and 532. Transistor 530 “steals”some current from transistor 540, thereby causing transistor 542 toreduce the amount of current I1.

From the foregoing it will be seen that when INA and INB are high enoughto reduce or eliminate the contribution of PMOS differential stage 200in FIG. 2, current I1 is increased to increase the effectiveness of NMOSdifferential stage 100. In this way, NMOS stage 100 can compensate forthe loss of the PMOS stage 200 contribution and maintain the operatingspeed of LVDS buffer 70. When INA and INB are not so high as to preventPMOS stage 200 from contributing, current I1 does not need to be solarge and is accordingly reduced.

The embodiment of current source I2 shown in FIG. 5 is conceptuallysimilar to what is shown in FIG. 4. The FIG. 5 circuit increases currentI2 when INA and INB are low (close to Vtn). This enables PMOS stage 200in FIG. 2 to operate more strongly when NMOS stage 100 is weak or unableto operate. The construction and operation of the FIG. 5 circuit are sosimilar to the FIG. 4 circuit that it is not believed necessary todescribe FIG. 5 in full detail. Analogous elements in FIGS. 4 and 5 havereference numbers that differ by 50. Thus element 510 in FIG. 4 isanalogous to element 560 in FIG. 5. Inverter 73 inverts the outputsignal of FCE 72 for application to the gates of PMOS transistors 552and 554 so that the FIG. 5 current source is turned on or off by thesame state of FCE 72 that turns the current sink of FIG. 4 on or off,respectively.

Preferred LVDS output buffer circuitry 600 in accordance with theinvention is shown in FIG. 6. A data signal (e.g., from logic circuitry(not shown, but typically conventional) on device 10) is applied tooutput buffer circuitry 600 via inverter 602. An LVDS output bufferenable signal (e.g., from a programmable FCE or from logic circuitry ondevice 10) is applied to circuitry 600 via lead 604. If the signal onlead 604 is a buffer-enabling signal (logic 1 in the depictedembodiment), NMOS transistor 606 is enabled to pass the data signal frominverter 602. The signal on lead 604 is inverted by inverter 608 andpassed by transistor 610 to be applied to one input terminal of each ofNOR gates 630 a and 630 b. Assuming that the signal on lead 604 is logic1, NOR gates 630 a and 630 b will be enabled by the resulting logic 0inputs to pass (in inverted form) the signals applied to their otherinput terminals. The output signal of transistor 610 is inverted by twosuccessive inverters 632 and 636 and applied to the gate of PMOStransistor 640. (Transistor 634 is connected in level-restoringrelationship to inverter 632.) Again assuming that the signal on lead604 is logic 1, the resulting logic 0 signal applied to the gate oftransistor 640 turns on that transistor.

The data signal passed by transistor 606 (assuming that transistor 606is turned on by the signal on lead 604) is inverted by successiveinverters 612 and 616 and by NOR gate 630 b. (Transistor 614 isconnected in level-restoring relationship to inverter 612.) The datasignal output by inverter 612 is also passed by transmission gate 620(which has a delay approximately equal to the delay of inverter 616) andinverted by NOR gate 630 a.

The output signal of NOR gate 630 a is applied to the gates of NMOStransistors 650 a and 652 a. The output signal of NOR gate 630 b isapplied to the gates of NMOS transistors 650 b and 652 b. Accordingly,when the data signal applied to inverter 602 is logic 0, transistors 650a and 652 a will be on and transistors 650 b and 652 b will be off. Onthe other hand, when the data signal applied to inverter 602 is logic 1,transistors 650 a and 652 a will be off and transistors 650 b and 652 bwill be on. The upper one of I/O pins 20 in FIG. 6 is connected betweenthe source terminal of transistor 652 b and the drain terminal oftransistor 652 a. The lower one of I/O pins 20 in FIG. 6 is connectedbetween the source terminal of transistor 650 a and the drain terminalof transistor 650 b.

From the foregoing it will be seen that a logic 0 data signal applied toinverter 602 connects lower I/O pin 20 to Vccn via resistor 660 a,transistor 640, resistor 660 b, and transistor 650 a. (Vccn is a powersupply voltage which may be higher than Vcc. For example, Vccn may be3.3 volts or 2.5 volts when Vcc is 1.8 volts.) The same logic 0 datasignal connects the upper I/O pin 20 to Vss via transistor 652 a andresistor 660 c. Accordingly, a logic 0 data signal causes current toflow from lower I/O pin 20 through the external LVDS circuit (includingresistor 670) to upper I/O pin 20.

A logic 1 data signal applied to inverter 602 connects upper I/O pin 20to Vccn via resistor 660 a, transistor 640, resistor 660 b, andtransistor 652 b, and connects lower I/O pin 20 to Vss via transistor650 b and resistor 660 c. Accordingly, a logic 1 data signal causescurrent to flow from upper I/O pin 20 through the external LVDS circuit(including resistor 670) to lower I/O pin 20. The network includingtransistors 650 and 652 therefore constitutes differential outputswitching circuitry configured to produce a pair of LVDS signals atterminals 20 in response to a single input signal applied to inverter602.

The circuitry that includes PMOS transistor 680 a acts like a capacitorto help reduce possible voltage excursions of the node at the lower endof resistor 660 b during transitions in the data signal applied toinverter 602 when all of transistors 650 and 652 may turn on briefly.The circuitry that includes PMOS transistor 680 b similarly acts like acapacitor to help reduce possible voltage excursions of the node at theupper end of resistor 660 c during transitions in the data signalapplied to inverter 602.

The LVDS output buffer construction shown in FIG. 6 has severalimportant advantages. In general, a circuit for supplying LVDS currentthrough resistor 670 tends to exhibit current variations due to changesin (1) Vccn, (2) temperature, and (3) the process by which device 10 wasmanufactured. However, the LVDS specification has relatively narrowranges for permissible output pin voltage difference Vod (250 to 450millivolts) and Voffset (1.125 to 1.375 volts). The circuitry shown inFIG. 6 is able to meet these output requirements despite variations ofthe types mentioned earlier in this paragraph.

The LVDS buffer shown in FIG. 6 is constructed as a ratioed circuitincluding (1) NMOS output transistors 650 and 652 which perform theactual switching, (2) PMOS transistor 640 as a current source, (3)resistors 660 a, 660 b, and 660 c to adjust the DC voltages at theoutput, and (4) capacitors 680 a and 680 b to aid in AC (alternatingcurrent or transient) performance. Output transistors 650 and 652 aresized so that their on-resistance is small compared to bias resistors660 b and 660 c. Resistor 660 a serves to increase the output resistanceof PMOS transistor 640, as well as providing for local series feedbackso that when the power supply varies, the change in the current is notas great as it otherwise would be. Resistors 660 a, 660 b, and 660 c arepreferably all N-plus type to track any variations due to temperature ormanufacturing process in NMOS output transistors 650 and 652. The DCvoltages at the output, as well as the standby current, are adjusted byadjusting the values of resistors 660 b and 660 c. The ratio between (1)the resistors 660 a and 660 b above transistors 650 and 652 and (2) theresistor 660 c below those transistors determines the Voffset range. Thetotal of these resistances (and resistor 670) determines the range ofVod and thus the range of current through resistor 670 (which gives thedifferential signaling). Because the DC voltages and currents are set bythe ratio of the resistors to the transistors, variations in process,temperature, and power supply voltages are rejected.

Capacitors 680 a and 680 b reduce charge-sharing effects when the outputswitches. The capacitors keep the nodes above and below output switchingtransistors 650 and 652 from drifting during switching and allow forfaster rise and fall times, as well as overall speed improvement in theforward delay of the entire output driver.

A disable function is implemented by using NOR gates 630 a and 630 b.When disabled, all the output transistors are off, as well as the PMOScurrent source (i.e., transistor 640). The disable control signal 604can either be set by an FCE in device 10, or it can be a logic signalrouted out to driver 600 so that the user can have the option ofenabling it.

An explanation of NMOS transistor 690 is as follows. All inverters,transistors, and NOR gates in FIG. 6 (except inverters 602 and 608)preferably use thick oxide. The reason for this is that these componentsmay see a voltage level Vccn which is higher than Vcc. As has beenmentioned, Vccn may be 3.3 volts or 2.5 volts when Vcc is 1.8 volts. Thethick oxide protects the device integrity for reliability reasons (dueto high voltage). A disadvantage of using thick oxide is that itslightly slows down the speed. Transistor 690 is added to speed up thelogic path using a “look ahead” technique. When the output of inverter602 switches from low to high, transistor 690 turns on and pulls downthe input of inverter 616, instead of waiting for the path that includeselements 606 and 612 to respond. When the output of inverter 602switches from high to low, the “look ahead” does not exist. Transistor690 simply turns off. The input of inverter 616 is pulled up only whenthe signal propagates via elements 606 and 612. But usually thelow-to-high transition is the worst case (i.e., the speed-limitingtransition), and the “look ahead” feature improves performance for thiscase.

FIG. 7 shows circuitry in accordance with the invention for allowing thetwo I/O pins 20 that are used together with LVDS output buffer 600 to bealternatively used individually with conventional single-conductor inputbuffers 50 or conventional single-conductor output buffers 60 (similarto similarly numbered elements in earlier FIGS.). Buffer 600 has anassociated FCE 601 (the output signal of which can produce the signal onlead 604 in FIG. 6). FCE 601 is programmable by the user of device 10 tocontrol whether or not LVDS output buffer 600 is enabled. Each ofbuffers 50 and 60 similarly has an associated FCE 52 or 62 programmableby the user to determine whether or not the associated buffer isenabled. If buffer 600 is enabled, then the I/O pins 20 shown in FIG. 7are used as a pair for LVDS, and all of the other buffers 50 and 60associated with those pins are disabled. On the other hand, if buffer600 is disabled, then any one of the other buffers 50/60 associated witheach of the I/O pins shown in FIG. 7 can be enabled to allow each pin tobe used individually as either a conventional (single-conductor) outputpin or a conventional (single-conductor) input pin.

FIG. 8 shows that the signal which selectively enables any of outputbuffers 60 or 600 can be a logical combination of the output signal ofan FCE 62 or 601 and an output enable signal from the logic of device10. This logical combination is produced by logic gate 700, which can bechosen to perform any desired logic function (e.g., a NOR function).

FIG. 9 shows that two I/O pins 20 can be used for either LVDS input(employing LVDS input buffer 70), LVDS output (employing LVDS outputbuffer 600), or conventional single-conductor input or output (employingbuffers 50/60). In effect, FIG. 9 shows that the circuitry shown inFIGS. 1 and 7 can be combined in relation to a given pair of I/O pins20.

Restating some of the foregoing in more generic terms, a programmablelogic device 10 in accordance with the invention includes logiccircuitry (“from logic”/“to logic” in FIGS. 1, 2, and 6–9) and a pair ofinterface terminals 20 for use in making connections to circuitry30/40/670 that is external to the programmable logic device. The devicefurther includes LVDS buffer circuitry 70/600 connected to both of theinterface terminals 20 and configured to exchange with the interfaceterminals a pair of signals which are respectively associated with thepair of interface terminals and which differ from one another in voltagein order to represent information in accordance with an LVDS standard.The LVDS buffer circuitry may be an input buffer 70 configured toreceive the pair of signals from interface terminals 20 and to produce asingle output signal (“to logic”) indicative of the information forapplication to the logic circuitry of device 10. Alternatively the LVDSbuffer circuitry may be an output buffer 600 configured to receive fromthe logic circuitry (“from logic”) of device 10 a single input signalindicative of the information and to produce the pair of signalsindicative of the information for application to interface terminals 20.

Device 10 may also include programmable function control circuitry72/601 configured to selectively enable the LVDS buffer circuitry70/600. Device 10 may further include single-conductor signaling buffercircuitry 50/60 connected to one of the interface terminals 20 andconfigured to exchange with the one of the interface terminals a singlesignal having voltages which represent data in accordance with asingle-conductor signaling standard. Programmable function controlcircuitry 52/62/72/601 may be provided for selectively enabling eitherthe LVDS buffer circuitry 70/600 or the single-conductor signalingbuffer circuitry 50/60 while disabling remaining ones of the LVDS buffercircuitry 70/600 and the single-conductor signaling buffer circuitry50/60.

FIG. 10 illustrates a programmable logic device 10 of this invention ina data processing system 802. Data processing system 802 may include oneor more of the following components: a processor 804; memory 806; I/Ocircuitry 808; and peripheral devices 810. These components are coupledtogether by a system bus 820 and are populated on a circuit board 830which is contained in an end-user system 840.

System 802 can be used in a wide variety of applications, such ascomputer networking, data networking, instrumentation, video processing,digital signal processing, or any other application where the advantageof using programmable or reprogrammable logic is desirable. Programmablelogic device 10 can be used to perform a variety of different logicfunctions. For example, programmable logic device 10 can be configuredas a processor or controller that works in cooperation with processor804. Programmable logic device 10 may also be used as an arbiter forarbitrating access to a shared resource in system 802. In yet anotherexample, programmable logic device 10 can be configured as an interfacebetween processor 804 and one of the other components in system 802. Itshould be noted that system 802 is only exemplary, and that the truescope and spirit of the invention should be indicated by the followingclaims.

Various technologies can be used to implement programmable logic devices10 providing the LVDS capabilities of this invention. For example,function control elements 52/62/72/601 and other FCEs can be SRAMs,DRAMs, first-in first-out (“FIFO”) memories, EPROMs, EEPROMS, functioncontrol registers (e.g., as in Wahlstrom U.S. Pat. No. 3,473,160),ferro-electric memories, fuses, antifuses, or the like. From the variousexamples mentioned above it will be seen that this invention isapplicable to both one-time-only programmable and reprogrammabledevices.

It will be understood that the foregoing is only illustrative of theprinciples of the invention, and that various modifications can be madeby those skilled in the art without departing from the scope and spiritof the invention. For example, the numbers and types of otherconventional input and output buffers 50/60 that can make alternativeuse of the I/O pins 20 associated with LVDS buffers 70/600 in accordancewith the invention can be varied as desired.

1. Buffer circuitry configured to produce from an input signalindicative of information a pair of output signals that indicate theinformation by the relative polarity of their voltages comprising: asource of relatively high voltage; a source of relatively low voltage;differential switching circuitry configured to produce the outputsignals in response to the input signal; first resistor circuitryconnected in series between the source of relatively high voltage andthe differential switching circuitry; and second resistor circuitryconnected in series between the differential switching circuitry and thesource of relatively low voltage, wherein: connections between thesources, the differential switching circuitry, and the first and secondresistor circuitries are such that when external circuitry is connectedin series between the two output signals, current flows in series fromthe source of relatively high voltage through the first resistorcircuitry, through the external circuitry, through the second resistorcircuitry, to the source of relatively low voltage; and at least one ofthe first and second resistor circuitries includes a transistor coupledin series with a first resistor and a second resistor and is configuredto increase in resistance in response to an increase in voltagedifference between the source of relatively high voltage and the sourceof relatively low voltage.
 2. The buffer circuitry defined in claim 1wherein transistors in the differential switching circuitry and thefirst and second resistor circuitries are configured to have similarchanges in resistance due to variations in a manufacturing process formaking an integrated circuit including those circuitries.
 3. The buffercircuitry defined in claim 1 wherein transistors in the differentialswitching circuitry and the first and second resistor circuitries areconfigured to have similar changes in resistance due to changes intemperature of an integrated circuit including those circuitries.
 4. Thebuffer circuitry defined in claim 1 further comprising: circuitryconfigured to turn off the differential switching circuitry when it isnot desired to use the output signals.
 5. A programmable logic devicehaving programmable logic circuitry comprising: the buffer circuitrydefined in claim 1, wherein the input signal is provided to the buffercircuitry by the logic circuitry of the device.
 6. A digital processingsystem comprising: processing circuitry; a memory coupled to saidprocessing circuitry; and a logic device as defined in claim 5 coupledto the processing circuitry and the memory.
 7. A printed circuit boardon which is mounted a logic device as defined in claim
 5. 8. The printedcircuit board defined in claim 7 further comprising memory circuitrymounted on the printed circuit board and coupled to the logic device. 9.The printed circuit board defined in claim 7 further comprisingprocessing circuitry mounted on the printed circuit board and coupled tologic device.
 10. A programmable logic device comprising: programmablelogic circuitry; the buffer circuitry defined in claim 1, wherein theinput signal is provided to the buffer circuitry by the programmablelogic circuitry, and wherein the pair of output signals comprises afirst output signal and a second output signal; first and secondinterface terminals configured for use in making connections between theprogrammable logic circuitry and the external circuitry, wherein thebuffer circuitry is connected to the first and second interfaceterminals and configured to selectively provide the first and secondinterface terminals with the respective first and second output signals;and input buffer circuitry configured for use in processing first andsecond input signals that indicate input information by the relativepolarities of their voltages, wherein the input buffer circuitry isconnected to the first and second interface terminals and configured toselectively receive from the first and second interface terminals therespective first and second input signals for providing a third outputsignal indicative of the input information to the programmable logiccircuitry.
 11. A method for receiving an input signal indicative ofinformation and for providing a pair of output signals on a pair of pinsthat indicate the information by the relative polarity of their voltagescomprising: coupling a first of the two pins to a source of relativelyhigh voltage via first resistor circuitry and coupling a second of thetwo pins to a source of relatively low voltage via second resistorcircuitry when the input signal is at a first logic level; coupling thesecond of the two pins to the source of relatively high voltage via thefirst resistor circuitry and coupling the first of the two pins to thesource of relatively low voltage via the second resistor circuitry whenthe input signal is at a second logic level; and configuring atransistor in one of the first and second resistor circuitries toincrease in resistance in response to an increase in voltage differencebetween the source of relatively high voltage and the source ofrelatively low voltage, wherein said transistor is coupled in serieswith a first resistor and a second resistor.
 12. The method of claim 11further comprising disposing external circuitry in series between thetwo pins.
 13. The method of claim 11 wherein the coupling furthercomprises using switching circuitry.
 14. The method of claim 13 whereinthe switching circuitry and the first and second resistor circuitriesinclude transistors, the method further comprising configuring thetransistors to have similar changes in resistance due to variations in amanufacturing process for making an integrated circuit including thosecircuitries.
 15. The method of claim 13 wherein the switching circuitryand the first and second resistor circuitries include transistors, themethod further comprising configuring the transistors to have similarchanges in resistance due to changes in temperature of an integratedcircuit including those circuitries.
 16. The method of claim 13 furthercomprising disabling the switching circuitry when it is not desired toprovide the pair of output signals.